Propylene oxide is widely used as precursor for preparing polyether polyols, which upon reaction with polyisocyanate compounds yield polyurethanes. Typically, methods for preparing polyether polyols involve reacting a starting compound having a plurality of active hydrogen atoms with propylene oxide, optionally together with one or more other alkylene oxides like ethylene oxide or butylene oxide. Suitable starting compounds include polyfunctional alcohols, generally containing 2 to 6 hydroxyl groups. Examples of such alcohols are glycols, glycerol, pentaerythritol, trimethylolpropane, triethanolamine, sorbitol, mannitol, etc. Usually a strong base like potassium hydroxide is used as a catalyst in this type of reaction.
The quality of the propylene oxide used to prepare the polyether polyol has significant impact on the quality of the polyurethane foams eventually obtained, especially when these foams are high resilience flexible polyurethane foams. Particularly the presence of poly(propylene oxide) is known to cause undesired effects in the polyurethane foam formation. Examples of such undesired effects are the occurrence of blow holes, low foam rise and even collapse of the foam formed. Particularly, in moulding applications, the presence of poly(propylene oxide) in the propylene oxide used for preparing the starting polyether polyol may cause problems in terms of quality of the polyurethane foam. The presence of poly(propylene oxide) in propylene oxide used for preparing a polyether polyol for making slabstock polyurethane foams, may be less problematic.
In producing slabstock polyurethane foams, slabs of polyurethane foam are produced continuously or discontinuously as semi-finished products and are finally cut to the required size and shape. The characteristic feature of moulded polyurethane foams in which they fundamentally differ from slabstock polyurethane foams, is the manner of their production. This proceeds by reaction of the polyurethane raw materials in moulds. The finished moulded product no longer has to be cut to the required size and shape. For a further description of the differences between slabstock and moulded polyurethane foams, reference is made to handbooks on polyurethane foams, such as “Polyurethane Handbook/Chemistry—Raw Materials—Processing—Application—Properties” by Günter Oertel (Carl Hanser Verlag, Munich 1985).
It has appeared in practice that, in general, where propylene oxide is to be used to prepare a polyether polyol for making moulded polyurethane foams, no more than 1 ppm of poly(propylene oxide) should be present in said propylene oxide. If more propylene oxide is present, one or more of the above-mentioned undesired effects may occur when making the foam. On the other hand, where propylene oxide is to be used to prepare a polyether polyol for making slabstock polyurethane foams, in general, up to 3 ppm of poly(propylene oxide) may be present in the propylene oxide.
Methods for manufacturing propylene oxide are well known in the art. Commercial production normally takes place via the chlorohydrin process or via the hydroperoxide process. In the latter process propene is reacted with an organic hydroperoxide. This hydroperoxide is either tert-butyl hydroperoxide or ethylbenzene hydroperoxide. In the first case tert-butyl alcohol is formed as a co-product (to be further converted into methyl tert-butyl ether), in the second case styrene is formed as the co-product. In the chlorohydrin process chlorine, propene and water are reacted to form propylene chlorohydrin, which is subsequently dehydrochlorinated with calcium hydroxide to form propylene oxide. For the purpose of the purification of propylene oxide it is immaterial which preparation route is used. Namely, in all processes poly(propylene oxide) is formed in undesirably high quantities. Moreover, it is known (e.g. from U.S. Pat. No. 4,692,535) that high molecular weight poly(propylene oxide) may be formed during storage or transport, for example upon contact with a metal and/or metal oxide, such as metal oxide of carbon steel.
One method for purification of propylene oxide by membrane separation is known from U.S. Pat. No. 5,248,794. According to this method, suitable membranes are poly(vinylidene fluoride) and poly(acrylonitrile) polymeric membranes. Such polymers are commonly used in the art as materials for ultrafiltration membranes. See for example Table II.12 at page 56 of “Basic Principles of Membrane Technology”, Marcel Mulder, second edition, Kluwer Academic Publishers, 1996. In said Table II.12, poly(vinylidene fluoride) and poly(acrylonitrile), but also polysulfone and cellulose esters, are mentioned as examples of polymers for ultrafiltration membranes. Therefore, the membranes disclosed in U.S. Pat. No. 5,248,794 are ultrafiltration membranes. Ultrafiltration membranes are porous membranes which have an average pore size greater than 5 nm.
In Table 2 of Example 1 of U.S. Pat. No. 5,248,794, poly(propylene oxide) separation results are mentioned for some different types of membranes. The poly(propylene) membranes were not considered suitable. Only the poly(vinylidene fluoride) and poly(acrylonitrile) ultrafiltration membranes were considered suitable for the purpose of U.S. Pat. No. 5,248,794. One way of determining the suitability of a membrane for separating poly(propylene oxide) (PPO) from propylene oxide, is by calculating the PPO rejection, as follows:PPO rejection (%)=(1−([PPO]p/[PPO]f))*100wherein [PPO]p is the poly(propylene oxide) concentration in the permeate and [PPO]f is the poly(propylene oxide) concentration in the feed. Where in the present specification reference is made to PPO rejection, the PPO rejection defined in the above way is meant.
In Example 1 of U.S. Pat. No. 5,248,794, using poly(propylene) membranes resulted in a negative PPO rejection. Further, using poly(vinylidene fluoride) and poly(acrylonitrile) ultrafiltration membranes resulted in relatively low PPO rejections (<30%). The poly(propylene oxide) concentrations in the permeates as mentioned in Table 2 of U.S. Pat. No. 5,248,794, are so high that these permeates cannot be used in the production of moulded polyurethane foams and neither in the production of slabstock polyurethane foams.
Further, in Example 2 of U.S. Pat. No. 5,248,794, the same poly(acrylonitrile) membrane as used in Example 1, was tested with a continuous flow of unfiltered propylene oxide over a period of 86 days. The results are listed in Table 3 of U.S. Pat. No. 5,248,794. From this it appears that in the course of time, the PPO rejection increases from a value of only 31% (on day 1) to a maximum of 100% (on days 52 and 55) and then decreases again to a value of only 67% (on day 86). Therefore, the membrane used is disadvantageous in that there is a relatively long waiting period (of about 52 days) before permeate is produced of which the quality is such that it can be used in the production of moulded polyurethane foams and slabstock polyurethane foams. Further, after day 55, permeate of inferior quality is produced again. Besides a low and unstable PPO rejection in time, another disadvantage is that the permeate flow is also unstable. In said Example 2, the permeate flow goes from a maximum value of 90 ml/min. at the beginning of the experiment, to a value of only 37 ml/min. at the end. This indicates that the membrane, being a porous poly(acrylonitrile) ultrafiltration membrane, gets clogged or plugged in the course of time.
In summary, both the selectivity of and the flow (flux) through the membrane used in Example 2 of U.S. Pat. No. 5,248,794 are relatively low and unstable in time. The volume or mass “flux” (volume or mass “permeation rate”) is defined as the volume or mass flowing through the membrane per unit area and time (expressed in l/h/m2 and kg/h/m2, respectively). The permeability of a membrane is defined as the flux through the membrane per unit pressure, and is expressed in l/h/m2/bar or kg/h/m2/bar. When performing the process of U.S. Pat. No. 5,248,794, a strong fluctuation of separation results in time is obtained. This is disadvantageous in that it is difficult to predict whether or not a certain specification, for example a maximum poly(propylene oxide) concentration in PO permeate, can be met on a certain day.
A further method for improving the quality of propylene oxide by membrane separation is known from GB-A-2348200. In this method, liquid propylene oxide is subjected to a treatment using a ceramic ultrafiltration (porous) membrane under such conditions that the amount of poly(propylene oxide) is reduced. In the Example of GB-A-2348200, a ceramic ultrafiltration membrane having a pore size of 6 nm was used in separating poly(propylene oxide) from propylene oxide. In said Example, it is stated that it was found that said membrane removed about 50% poly(propylene oxide). More in particular, it appears that in said Example, the poly(propylene oxide) concentration in the propylene oxide feed was 1.03 mg/l and that the poly(propylene oxide) concentration in the permeate was still 0.54 mg/l. This corresponds with a PPO rejection of only 48%.
In the above-discussed prior art methods for removing poly(propylene oxide) from propylene oxide by membrane separation, ultrafiltration membranes are used. Ultrafiltration is a pressure difference driven membrane filtration technique, wherein porous membranes are used which have an average pore size greater than 5 nm. One of the disadvantages of using ultrafiltration membranes as discussed above, is that the membranes foul during operation (membrane pores getting clogged or plugged) and have eventually to be taken out of operation for cleaning purposes. This will severely decrease the separation efficiency in time. A further disadvantage of using polymeric ultrafiltration membranes, is that they swell. Swelling has an effect on the pore size and results in permeability and selectivity instability.
In summary, from the above discussion of prior art methods for removing poly(propylene oxide) from propylene oxide by membrane separation, it appears that there is still a need in the art for a process using a membrane which, during a relatively long period of time, can separate poly(propylene oxide) from propylene oxide, in a stable way, both in terms of a stable PPO rejection and in terms of a stable permeate flow. In addition, this constant, stable PPO rejection should be sufficiently high such that the permeate produced at any time, can be used for example in the production of moulded polyurethane foams and/or slabstock polyurethane foams.